1. Field of the Invention
The present invention relates to lithium batteries. More particularly, the present invention relates to organic electrolyte solutions using monomer compounds which can be electrografted.
2. Description of the Related Art
As portable electronic devices, such as video cameras, cellular phones, and notebook PCs, become lighter and have higher performance, much research into batteries as driving power sources for the devices has been conducted. In particular, re-chargeable (secondary) lithium batteries have been actively studied because they have energy densities (per unit weight) three times greater than those of conventional lead storage batteries, nickel-cadmium batteries, nickel hydrogen batteries, nickel zinc batteries, etc. In addition, lithium secondary batteries can be rapidly re-charged.
Conventional lithium batteries are operated at high operating voltages, and thus, conventional aqueous electrolyte solutions cannot be used due to the vigorous reaction of the aqueous solution with the lithium used as the anode. In this regard, organic electrolyte solutions obtained by dissolving lithium salts in organic solvents are generally used in lithium batteries. In particular, organic solvents having high ion conductivity, high dielectric constants, and low viscosities have been used. However, it is difficult to obtain a single organic solvent having all of these properties, and thus, mixed solvents have been proposed, for example an organic solvent including a solvent with a high dielectric constant and another organic solvent with low viscosity.
When a carbonate-based, non-aqueous polar solvent is used in a lithium secondary battery, excess charge occurs due to a reaction between the carbon of the anode and the electrolyte solution during initial charging. Such an irreversible reaction forms a passivation layer, such as a solid electrolyte interface (SEI) film, on the surface of the anode. The SEI film prevents further decomposition of the electrolyte solution and maintains stable charging/discharging. The SEI film also serves as an ion tunnel through which only lithium ions pass. In general, organic solvents solvate lithium ions, and are cointercalated with lithium ions into a carbon anode during battery charging/discharging. However, SEI films allow only lithium ions to pass, thereby preventing the cointercalation of organic solvents with lithium ions into the carbon anode. This prevents degradation of the anode structure caused by the cointercalation of solvents and lithium ions during battery charging/discharging.
However, the SEI film gradually cracks and delaminates from the surface of the electrode due to volumetric expansion and shrinkage of the active material during repeated charging/discharging. As a result, the electrolyte directly contacts the active material, causing continuous decomposition of the electrolyte. Once the SEI film cracks, the crack continuously extends during charging/discharging, thereby degrading the active material. In particular, when the active material contains a metal, such as silicon, active material degradation worsens due to large volumetric changes during charge/discharge cycles. Furthermore, repeated volumetric shrinkage and expansion of the active material causes agglomeration of silicon particles.